.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\" Modified Sat Jul 24 17:51:42 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Tue Aug 17 11:42:20 1999 by Ariel Scolnicov (ariels@compugen.co.il)
.TH SYSCONF 3  2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
sysconf \- get configuration information at run time
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "long sysconf(int " "name" );
.fi
.SH DESCRIPTION
POSIX allows an application to test at compile or run time
whether certain options are supported, or what the value is
of certain configurable constants or limits.
.PP
At compile time this is done by including
.I <unistd.h>
and/or
.I <limits.h>
and testing the value of certain macros.
.PP
At run time, one can ask for numerical values using the present function
.BR sysconf ().
One can ask for numerical values that may depend
on the filesystem in which a file resides using
.BR fpathconf (3)
and
.BR pathconf (3).
One can ask for string values using
.BR confstr (3).
.PP
The values obtained from these functions are system configuration constants.
They do not change during the lifetime of a process.
.\" except that sysconf(_SC_OPEN_MAX) may change answer after a call
.\" to setrlimit( ) which changes the RLIMIT_NOFILE soft limit
.PP
For options, typically, there is a constant
.B _POSIX_FOO
that may be defined in
.IR <unistd.h> .
If it is undefined, one should ask at run time.
If it is defined to \-1, then the option is not supported.
If it is defined to 0, then relevant functions and headers exist,
but one has to ask at run time what degree of support is available.
If it is defined to a value other than \-1 or 0, then the option is
supported.
Usually the value (such as 200112L) indicates the year and month
of the POSIX revision describing the option.
Glibc uses the value 1
to indicate support as long as the POSIX revision has not been published yet.
.\" and 999 to indicate support for options no longer present in the latest
.\" standard. (?)
The
.BR sysconf ()
argument will be
.BR _SC_FOO .
For a list of options, see
.BR posixoptions (7).
.PP
For variables or limits, typically, there is a constant
.BR _FOO ,
maybe defined in
.IR <limits.h> ,
or
.BR _POSIX_FOO ,
maybe defined in
.IR <unistd.h> .
The constant will not be defined if the limit is unspecified.
If the constant is defined, it gives a guaranteed value, and
a greater value might actually be supported.
If an application wants to take advantage of values which may change
between systems, a call to
.BR sysconf ()
can be made.
The
.BR sysconf ()
argument will be
.BR _SC_FOO .
.SS POSIX.1 variables
We give the name of the variable, the name of the
.BR sysconf ()
argument used to inquire about its value,
and a short description.
.PP
First, the POSIX.1 compatible values.
.\" [for the moment: only the things that are unconditionally present]
.\" .TP
.\" .BR AIO_LISTIO_MAX " - " _SC_AIO_LISTIO_MAX
.\" (if _POSIX_ASYNCHRONOUS_IO)
.\" Maximum number of I/O operations in a single list I/O call.
.\" Must not be less than _POSIX_AIO_LISTIO_MAX.
.\" .TP
.\" .BR AIO_MAX " - " _SC_AIO_MAX
.\" (if _POSIX_ASYNCHRONOUS_IO)
.\" Maximum number of outstanding asynchronous I/O operations.
.\" Must not be less than _POSIX_AIO_MAX.
.\" .TP
.\" .BR AIO_PRIO_DELTA_MAX " - " _SC_AIO_PRIO_DELTA_MAX
.\" (if _POSIX_ASYNCHRONOUS_IO)
.\" The maximum amount by which a process can decrease its
.\" asynchronous I/O priority level from its own scheduling priority.
.\" Must be nonnegative.
.TP
.BR ARG_MAX " - " _SC_ARG_MAX
The maximum length of the arguments to the
.BR exec (3)
family of functions.
Must not be less than
.B _POSIX_ARG_MAX
(4096).
.TP
.BR CHILD_MAX " - " _SC_CHILD_MAX
The maximum number of simultaneous processes per user ID.
Must not be less than
.B _POSIX_CHILD_MAX
(25).
.TP
.BR HOST_NAME_MAX " - " _SC_HOST_NAME_MAX
Maximum length of a hostname, not including the terminating null byte,
as returned by
.BR gethostname (2).
Must not be less than
.B _POSIX_HOST_NAME_MAX
(255).
.TP
.BR LOGIN_NAME_MAX " - " _SC_LOGIN_NAME_MAX
Maximum length of a login name, including the terminating null byte.
Must not be less than
.B _POSIX_LOGIN_NAME_MAX
(9).
.TP
.BR NGROUPS_MAX " - " _SC_NGROUPS_MAX
Maximum number of supplementary group IDs.
.TP
.BR "" "clock ticks - " _SC_CLK_TCK
The number of clock ticks per second.
The corresponding variable is obsolete.
It was of course called
.BR CLK_TCK .
(Note: the macro
.B CLOCKS_PER_SEC
does not give information: it must equal 1000000.)
.TP
.BR OPEN_MAX " - " _SC_OPEN_MAX
The maximum number of files that a process can have open at any time.
Must not be less than
.B _POSIX_OPEN_MAX
(20).
.TP
.BR PAGESIZE " - " _SC_PAGESIZE
Size of a page in bytes.
Must not be less than 1.
.TP
.BR PAGE_SIZE " - " _SC_PAGE_SIZE
A synonym for
.BR PAGESIZE / _SC_PAGESIZE .
(Both
.BR PAGESIZE
and
.BR PAGE_SIZE
are specified in POSIX.)
.TP
.BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
The number of repeated occurrences of a BRE permitted by
.BR regexec (3)
and
.BR regcomp (3).
Must not be less than
.B _POSIX2_RE_DUP_MAX
(255).
.TP
.BR STREAM_MAX " - " _SC_STREAM_MAX
The maximum number of streams that a process can have open at any
time.
If defined, it has the same value as the standard C macro
.BR FOPEN_MAX .
Must not be less than
.B _POSIX_STREAM_MAX
(8).
.TP
.BR SYMLOOP_MAX " - " _SC_SYMLOOP_MAX
The maximum number of symbolic links seen in a pathname before resolution
returns
.BR ELOOP .
Must not be less than
.B _POSIX_SYMLOOP_MAX
(8).
.TP
.BR TTY_NAME_MAX " - " _SC_TTY_NAME_MAX
The maximum length of terminal device name,
including the terminating null byte.
Must not be less than
.B _POSIX_TTY_NAME_MAX
(9).
.TP
.BR TZNAME_MAX " - " _SC_TZNAME_MAX
The maximum number of bytes in a timezone name.
Must not be less than
.B _POSIX_TZNAME_MAX
(6).
.TP
.BR _POSIX_VERSION " - " _SC_VERSION
indicates the year and month the POSIX.1 standard was approved in the
format
.BR YYYYMML ;
the value
.B 199009L
indicates the Sept. 1990 revision.
.SS POSIX.2 variables
Next, the POSIX.2 values, giving limits for utilities.
.TP
.BR BC_BASE_MAX " - " _SC_BC_BASE_MAX
indicates the maximum
.I obase
value accepted by the
.BR bc (1)
utility.
.TP
.BR BC_DIM_MAX " - " _SC_BC_DIM_MAX
indicates the maximum value of elements permitted in an array by
.BR bc (1).
.TP
.BR BC_SCALE_MAX " - " _SC_BC_SCALE_MAX
indicates the maximum
.I scale
value allowed by
.BR bc (1).
.TP
.BR BC_STRING_MAX " - " _SC_BC_STRING_MAX
indicates the maximum length of a string accepted by
.BR bc (1).
.TP
.BR COLL_WEIGHTS_MAX " - " _SC_COLL_WEIGHTS_MAX
indicates the maximum numbers of weights that can be assigned to an
entry of the
.B LC_COLLATE order
keyword in the locale definition file.
.TP
.BR EXPR_NEST_MAX " - " _SC_EXPR_NEST_MAX
is the maximum number of expressions which can be nested within
parentheses by
.BR expr (1).
.TP
.BR LINE_MAX " - " _SC_LINE_MAX
The maximum length of a utility's input line, either from
standard input or from a file.
This includes space for a trailing
newline.
.TP
.BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
The maximum number of repeated occurrences of a regular expression when
the interval notation
.B \e{m,n\e}
is used.
.TP
.BR POSIX2_VERSION " - " _SC_2_VERSION
indicates the version of the POSIX.2 standard in the format of
YYYYMML.
.TP
.BR POSIX2_C_DEV " - " _SC_2_C_DEV
indicates whether the POSIX.2 C language development facilities are
supported.
.TP
.BR POSIX2_FORT_DEV " - " _SC_2_FORT_DEV
indicates whether the POSIX.2 FORTRAN development utilities are
supported.
.TP
.BR POSIX2_FORT_RUN " - " _SC_2_FORT_RUN
indicates whether the POSIX.2 FORTRAN run-time utilities are supported.
.TP
.BR _POSIX2_LOCALEDEF " - " _SC_2_LOCALEDEF
indicates whether the POSIX.2 creation of locales via
.BR localedef (1)
is supported.
.TP
.BR POSIX2_SW_DEV " - " _SC_2_SW_DEV
indicates whether the POSIX.2 software development utilities option is
supported.
.PP
These values also exist, but may not be standard.
.TP
.BR "" " - " _SC_PHYS_PAGES
The number of pages of physical memory.
Note that it is possible
for the product of this value and the value of
.B _SC_PAGESIZE
to overflow.
.TP
.BR "" " - " _SC_AVPHYS_PAGES
The number of currently available pages of physical memory.
.TP
.BR "" " - " _SC_NPROCESSORS_CONF
The number of processors configured.
See also
.BR get_nprocs_conf (3).
.TP
.BR "" " - " _SC_NPROCESSORS_ONLN
The number of processors currently online (available).
See also
.BR get_nprocs_conf (3).
.SH RETURN VALUE
The return value of
.BR sysconf ()
is one of the following:
.IP * 3
On error, \-1 is returned and
.I errno
is set to indicate the error
(for example,
.BR EINVAL ,
indicating that
.I name
is invalid).
.IP *
If
.I name
corresponds to a maximum or minimum limit, and that limit is indeterminate,
\-1 is returned and
.I errno
is not changed.
(To distinguish an indeterminate limit from an error, set
.I errno
to zero before the call, and then check whether
.I errno
is nonzero when \-1 is returned.)
.IP *
If
.I name
corresponds to an option,
a positive value is returned if the option is supported,
and \-1 is returned if the option is not supported.
.IP *
Otherwise,
the current value of the option or limit is returned.
This value will not be more restrictive than
the corresponding value that was described to the application in
.I <unistd.h>
or
.I <limits.h>
when the application was compiled.
.SH ERRORS
.TP
.B EINVAL
.I name
is invalid.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR sysconf ()
T}	Thread safety	MT-Safe env
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH BUGS
It is difficult to use
.B ARG_MAX
because it is not specified how much of the argument space for
.BR exec (3)
is consumed by the user's environment variables.
.PP
Some returned values may be huge; they are not suitable for allocating
memory.
.SH SEE ALSO
.BR bc (1),
.BR expr (1),
.BR getconf (1),
.BR locale (1),
.BR confstr (3),
.BR fpathconf (3),
.BR pathconf (3),
.BR posixoptions (7)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
